I've tried the silicone fix but it still leaks. I think the water gets into the seal itself at the top where it meets the back window, then works its way down inside the seal cavity to the bottom where it simply leaks out into the boot over the lip. Whenever I have found water after a drive in rain, or washing the car, there is always a small pool of water on the metalwork just inside the seal - bottom left hand side.
Ordered a new seal from Lexus today, £74, and going to fit it next week. Watch here for an update.
I did look at the tail light but that doesn't seem to be where the water gets in as that section of the boot is completely dry inside.